The invention relates to the construction and operation of superconducting electric machines, including superconducting electric motors.
There are a wide variety of electric motors, including linear and rotary motors. Among the more popular rotary electric motors are AC induction and AC synchronous motors. With both motors, the manner in which the motor rotates involves the interaction of magnetic fields of the rotor and the stator.
For the AC induction motor, the stator windings are usually connected to a supply in one or three phase form. By applying a voltage across the winding, a radial rotating magnetic field is formed. The rotor has layers of conductive strands along its periphery. The strands are generally short-circuited to form conductive closed loops. The rotating magnetic fields produced by the stator induce a current into the conductive loops of the rotor. Once that occurs, the magnetic field causes forces to act on the current carrying conductors, which results in a torque on the rotor.
The advantage and simplicity of the AC induction motor is that the currents in the rotor do not have to be supplied by commutator, like they do in a DC motor. The velocity of the rotating magnetic field of the stator can be calculated with the formula below:V=πrf/p 
where:                p is the number of poles;        r is the radius of the air gap; and        f is the frequency.The rotor reacts to the magnetic field, but does not travel at the same speed. The speed of the rotor actually lags behind the speed of the magnetic field. The term “slip” is generally used to quantify the slower speed of the rotor relative to the magnetic field. The rotor is not locked into any position and, therefore, will continue to slip throughout the motion. The amount of slip increases proportionally with increases in load. More recently, induction motors are being controlled by AC variable speed drives (inverters). These drives control the frequency of the AC supply fed to the windings, making the induction motor a growing competitor in the controlled velocity market, where the DC motor previously dominated. However, one needs to insure that the motor is inverter rated before coupling the two together. The problem of slip will still exist, unless velocity feedback is provided.        
AC synchronous motors have a stator very similar to that of the AC induction motor. An AC synchronous motor stator includes slots along its periphery within which windings are placed. The quantity of windings and slots is determined in part by the number of phases (usually 3 or 1) and the number of poles (usually 2 or 4). The stator produces a rotating magnetic field that is proportional to the frequency supplied. As was the case with the AC induction motor, the speed of the rotating magnetic field in an AC synchronous motor, is calculated with the formula V=πrf/p.
The main difference between the synchronous motor and the induction motor is that the rotor of this motor travels at the same speed as the rotating magnetic field. This is possible because the magnetic field of the rotor is no longer induced. The rotor either has permanent magnets or dc excited currents, which are forced to lock into a certain position when confronted with another magnetic field.
Thus, the problem with slip and speed variation with varying loads is eliminated.
Recently, efforts have been ongoing in applying cryogenic technology to electric machines, including both induction and synchronous electric motors. The use of superconducting windings in these machines has resulted in a significant increase in the field magnetomotive forces generated by the windings and increased flux and power densities of the machines.